Characterizing Nitrogen Sites in Nitrogen-Doped Reduced Graphene Oxide: A Combined Solid-State <sup>15</sup> N NMR, XPS, and DFT Approach
Gunwoo Kim, Jeongjae Lee, Tao Liu, Clare P. Grey
Abstract
Despite the potential applications in energy storage and conversion systems such as Li–oxygen batteries and fuel cells, the nature and distribution of doped nitrogen sites in reduced graphene oxides are still not well understood. In this work, we report a combined approach of 15N solid-state nuclear magnetic resonance (NMR) spectroscopy alongside the predominantly used X-ray photoelectron spectroscopy (XPS) to characterize the nitrogen environments in reduced graphene oxides. Application of 1H–15N low-power double-quantum cross polarization under fast magic angle spinning with the Carr–Purcell–Meiboom–Gill scheme shows selective detection of protonated sites with low-power radio-frequency irradiation. NMR shift calculations of a series of N-containing molecules and a graphene nanoflake model were performed to help interpreting the experimental data. This work demonstrates a powerful approach to identify and quantify the different nitrogen environments in doped graphene materials and can also be widely applied to similar graphitic carbon-based materials with other dopants.